1. Field of the Invention
The present invention relates to an activated sludge method, and more particularly to a method of removing nitrogen and phosphorus from waste water by using activated sludge including aerobic bacteria.
2. Description of the Invention
Various activated sludge processes for removing nitrogen and/or phosphorus from waste water by using an activated sludge including aerobic bacteria have been proposed for the purpose of protecting enclosed water areas such as lakes and bays from eutrophication.
One of the activated sludge processes for denitrification and phosphorus removal is illustrated in FIGS. 1 and 2. FIG. 1 shows a tank 10 which has a baffle plate 12 provided therein. The baffle plate 12 divides the tank 10 into a basin 14 for pre-treating the waste water by a small amount of the sludge contained therein and a basin 16 for treating the waste water by the sludge contained in the tank 10, with a mixed liquor consisting of the waste water and the sludge being formed in the basin 16. An air diffuser 18 is disposed at the bottom of the treatment basin 16 to aerate the mixed liquor held in the treatment basin 16. FIG. 2 shows a time schedule along which the mixed liquor is treated. As shown in FIG. 2, the mixed liquor is aerated in the treatment basin 16 by the air diffuser 18 over a period of three hours. In this aerobic condition, nitrification is accomplished in the mixed liquor. That is, ammonia nitrogen included in the mixed liquor is nitrified and changed into nitrate nitrogen. On the other hand, phosphorus included in the mixed liquor is ingested by the sludge bacteria, with organic material included in the mixed liquor also being consumed. Then, the mixed liquor is allowed to settle over a period of one hour so that it is separated into a supernatant liquid and the sludge. After the settling is started, the mixed liquor presents an anoxic condition so that due to the existence of the sludge bacteria, nitrate nitrogen is subjected to reduction while the organic material is subjected to oxidization, thereby causing the denitrification. After settling is finished, the supernatant liquid is discharged as an effluent from the treatment basin 16 over a period of two hours while a new waste water is introduced into the treatment basin 16 through the pre-treatment basin 14. When the supernatant liquid is discharged, a decanter 20 as shown in FIG. 1, which is well known in this field, is used. On the other hand, the settled sludge is partially drawn out from the treatment basin 16, if necessary and the remainder is reused for treating the new waste water. As is apparent from FIG. 2, a cycle consisting of the aerating, settling and discharging stages takes six hours and is repeated four times per day.
The activated sludge process as discussed above is unsatisfactory because the denitrification and the phosphorus removal are achieved only at a low percentage (about 60%).
JAPANESE PATENT PUBLICATION No. 56 (1981)-53435 discloses an activated sludge process wherein a cycle consisting of agitating and aerating stages is repeated in a single tank in which a mixed liquor consisting of waste water and activated sludge is contained, and the agitating time and the aerating time are controlled in such a manner that the ratio of nitrate nitrogen to ammonia nitrogen is maintained within a predetermined range, whereby the denitrification is achieved with high efficiency. However, this activated sludge method is also unsatisfactory because almost no phosphorus removal is obtained, although the denitrification is highly efficiently achieved.
In general, for highly efficient removal of phosphorus from waste water by use of the activated sludge, it is necessary to put the mixed liquor under the anaerobic condition so that the aerobic bacteria releases phosphorus into the mixed liquor in order to stay alive. This is because the aerobic bacteria which have been stressed to release phosphorus then excessively ingest phosphorus from the mixed liquor so that phosphorus is removed from the waste water with high efficiency.
"Water SA", Vol. 2, No. 3, July, 1976 discloses an activated sludge process for conducting denitrification and phosphorus removal at a relatively high efficiency. This activated sludge process can be explained by a flow chart as shown in FIG. 3. In FIG. 3, first, the waste water is fed into an anaerobic basin 22 in which the sludge is contained so that a mixed liquor consisting of the waste water and sludge, is formed. The mixed liquor is kept in the basin 22 until phosphorus is released from the sludge bacteria. The mixed liquor is then transferred to an anoxic basin 24 where denitrification is carried out. The mixed liquor is further transferred to an aerobic basin 26 where phosphorus is excessively ingested by the sludge bacteria while the remaining ammonia nitrogen is nitrified and changed into nitrate nitrogen. In order for the denitrification, a part of the aerated mixed liquor is returned from the aerobic basin 26 to the anoxic basin 24. Thereafter, the mixed liquor is transferred from the aerobic basin 26 to a settling basin 28 where it is separated into a supernatant liquid and the sludge. The supernatant liquid is discharged as an effluent while the settled sludge is recycled to the anaerobic basin 22.
This activated sludge process is satisfactory in that the denitrification and the phosphorus removal can be achieved with a relatively high efficiency, but it entails the drawback of requiring the four basins 22, 24, 26 and 28 each of which must be of large capacity because the quantity of the mixed liquor to be held in each basin changes. In short, this process requires a large and expensive plant.